OVULE (Fr. ovule, from ML. orufune little egg). The structure which in seed-plants tsper matophytes) becomes a seed. The name refers to the old mistaken belief that the ovule of a plant represents the egg of an animal, and that it is fertilized and so produces a new plant. The name has been so long in use that it is still re tained. although its real meaning makes it very inappropriate. Really it is a spore-case (sporan gium. q.v.), and is entirely sexless. In sperms (pines and their allies) the ovules are freely exposed, while in angiosperms ( flowering plants) they are inclosed ill the bulbous part (ovary) of the pistil, but in both eases their structure is the same. The central body of an ovule (Fig. I) is the nueellus, and enwrapping it there are one or two coats called the integu ments. At the apex of the nucellus the integu ments leave a small passageway (tnicropyle, 'lit tle gate') for the entrance of the pollen-tube (see FERTILIZATION) ; and near the base of the ovule they become indistinguishable from the nueellus, this region of blending being called the chataza. Frequently the ovule has a slender stalk, called the funiculus. Within the nueellus a single large spore (megaspore, q.v.) is formed, which still bears its old name embryo-sac. This spore is peculiar in not being shed from its sporangium (ovule), and this fact results in making a seed out of the ovule. It is the business of this spore
to germinate and produce a female plant (game tophyte), that is, a plant which produces eggs. This female plant thus imprisoned within the seed was long ago named the endosperm (Fig. 2). See SEED.
In the true flowering plants (angiosperms) the ovules are exceedingly variable as to num ber and position within the ovary, which may contain one or a very large number of ovules, and these may be attached to the wall of the ovary, in which case they usually occur in defi nite lines: or they may be found attached to a central axis which projects more or less into the ovary cavity. Three well-marked forms of ovules have been distinguished, and they are usually characteristic of great groups of plants. The most common form is the anatropous (in verted) ovule, in which the ovule has completely turned over and its funieulus (stalk) appears as a ridge along one side of it. the micropyle (apex of ovule) thus being directed toward the base of the funiculus. Another form is the en»lpylo tropous ovule, in which the body of the ovule itself lias become curved. the mieropyle thus being directed downward. The third form is the orthotropous ovule, in which there is no inversion or curving, but the axis of the ovule continues that of the funieulus, and the miernpyle is di rected upward.